DYNAMIC NEUTRON RADIOGRAPHY OF A COMBUSTION ENGINE J. Brunner1, A. Hillenbach2, E. Lehmann 3, B. Schillinger 1

1Technische Universität München, Garching, Germany; 2Institut Laue Langevin, Grenoble, France, 3Paul Scherrer Institut, Villigen, Switzerland Abstract: Dynamic neutron radiography is a non-destructive testing method, which made big steps in the last years. Depending on the neutron flux, the object and the detector a time resolution down to 50 ms is possible. In the case of repetitive processes the object can be synchronized with the detector and better statistics in the image can be obtained by adding radiographies of the same phase. By delaying the trigger signal a radiography movie can be composed with a time resolution down to 100 µs. A combustion engine is an ideal sample for the explained technique, because the motor block of metal is relatively easy to penetrate, while oil and fuel attenuate the thermal neutron beam much stronger. Various experiments were performed at ILL and PSI. Soon the tomography station ANTARES at FRM-II will be ready for measurements. Introduction: Imaging with neutrons works identically to the medical x-ray imaging. A sample is irradiated by a well defined beam with a small divergence and the transmitted neutron flux is measured by a position sensitive neutron detector. The greylevels in the radiography image correspond to the transmitted intensity of the object. While x-rays interact mainly with the electron shell of the atoms depending on the atomic number, with the number of electrons, neutrons interact with the atomic core and allow a distinction between many materials and even between different isotops of the same element. In Fig. 1 the neutron and the x-ray attenuation coefficients µρ are plotted as a function of the atomic number Z. Most metals as Al, Zn, and even steel show a large penetration depth for neutrons, while hydrogen strongly attenuates them. Thus, for example it is possible to inspect the propagation and the distribution of organic liquids as lubricants, fuel, water, oil in real engines.

Fig. 1:Mass attenuation coefficients µρ of neutrons versus x-rays against the atomic number (ENDF database) In a first approximation for neutrons as well as for x-rays the exponential attenuation law is valid: I(x) = I0·e-µx exponential attenuation law µρ= stot/amu=µ/ρ I(x) ... particle intensity after a distance x in material I0 … particle intensity at in the vacuum

�ρ … mass attenuation coefficient[cm2/g] � ... attenuation coefficient[1/cm] σtot ... total cross section[barn=10-24cm2]

ρ … material density[g/cm3] amu … atomical mass unit[= 1.66 · 10-24 g] As neutron source either a research reactor or a accelerator based spallation source can be used. At the new research reactor FRM-II in Munich a high flux tomography station is going into operation soon. A thermal or a cold neutron spectrum can be used with two different collimation ratios of L/D 400 and 800 and a neutron flux of 1.2⋅108n/cm2s and 3⋅107n/cm2s. L is the distance of the object from the diaphragm, D is the diameter of the diaphragm. The beam cross section at the sample position is 40cm x 40cm.

Fig. 2: The neutron tomography station ANTARES at the new research reactor FRM-II, Munich The transmitted neutrons are detected by a two dimensional position sensitive detector. It typically consists of a 200µm thick scintillator of Li6FZnS which emits visible green light that is recorded by a sensitive CCD camera. In the Li6 the nuclear reaction takes place: 6Li+n => 3H+4He+4.79 MeV The reaction energy of 4.79 MeV of the products tritium and helium is released in the common scintillation material ZnS. If a neutron is detected in the average a light spot of 100µm appears on the scintillator. This limits at the moment the spatial resolution of a neutron radiography. The measuring time for one radiography will be typically in the order of 50 ms. In combination with high frame rate cameras real time movies can be recorded. In the case of repetitive processes the object can be synchronized with the detector. Thus, adding of such a radiography series of the same phase leads to a remarkable reduction of the noise level. By delaying the trigger signal for the camera a radiography movie can be composed with a time resolution down to 100 µs. Such a short gating time can be realized with an intensified CCD camera. Results: Before the FRM-II went into operation a series of experiments were carried out at the Paul Scherrer Institute, Switzerland and the Institute Laue Langevin, France in collaboration with the local imaging groups with the goal to determine and improve the image quality for dynamical radiography. Running combustion engines seemed to be interesting samples in order to test the method. A running combustion engine was observed for the first time at its work with neutrons at NEUTRA, PSI. The engine is an four stroke model air craft engine of 6.5cm3 displacement volume and 0.5 kW power. Running with nitro fuel it does not run less slowly than 4800 rpm. A cycle lasts 12.5 ms and was split up in 50 images of 250µs exposure time. 4000 images of the same phase are added up in order to improve the statistics for one single image(Fig. 3). By changing the phase of the camera trigger (by steps of 250µs) a movie is composed. The spatial resolution is about 0.15mm.

Fig. 3: Dynamic neutron radiography of a running model aircraft engine (left) at 4800 rpm, t= 250µs

At the Neutrograph experiment at ILL the very high neutron flux of 3⋅109n/cm2s is available for neutron imaging. Together with the group of A. Hillenbach a motorbike(Fig. 4, Fig. 5) and a four cylinder car engine(Fig. 6) was investigated. Both four stroke engines were pulled by an electric motor in order to avoid exhaust gases and complex cooling loops.

Fig. 4, Fig. 5: Two images of the running cycle of a motorbike at 800 rpm, 250µs with 60 images accumulated

Fig. 6: Piston cooling of a four stroke car combustion engine by an oil beam can be observed well by neutron radiography, t=200µs with 120 accumulations, image size is 24cm x 24cm This dynamic neutron investigations were performed with an detector system from the PSI. The PSI detector consisted of a peltier cooled CCD camera using a multi channel plate as an image intensifier coupled on the CCD-chip with fiber optics. With this equipment and the high flux thermal neutron beam an extremely high detection sensitivity was possible. The necessary fast triggering is done by the image intensifier. Since the scintillator decay time to 10% is 85 µs, that is the order of magnitude for the limit of the time resolution. Another result was the dynamical visualisation of an injection cloud. Radiographies of a high pressure diesel injection nozzle were taken. The attenuation of the injected liquid is about 0.1%. In the false color image Fig. 7 the small attenuation of the injection liquid cloud is clearly visible.

Fig. 7: Common Rail Diesel injection nozzle (500 bar) in action, 5000 neutron radiography frames of 100µs added by software, delay to the trigger, 600µs, 700µs, 800 µs, image size is 6.2cm x 8.2cm Discussion: The spatial resolution is limited at the moment in the best case at 100µm determined by the scintillator resolution. At ILL real time inspections down to a few milliseconds are possible. In the case of repetitive processes stroboscopic imaging improves the time resolution down to 50µs with a signal to noise ratio of 0.1%. This method has opened access to a new area of inspection. At such high flux experiments two effects limit the investigation have to be considered: the activation of the object during the irradiation and the degradation of the scintillator material. Regarding the first problem, objects containing cobalt or silver should be avoided, other materials decay in reasonable time. The next step will be an injection observation in a running engine under real conditions. Conclusions: The obtained result should be a starting point to contribute to the solution of many NDT problems as fuel injection, lubricant propagation an distribution, liquid to gas phase transitions in engines or mechanical stability and operation control. At FRM II the high performance neutron tomography station ANTARES will soon open the door to everybody looking for such kind of inspections. References: B. Schillinger, Doktorarbeit 1999, “Neue Entwicklungen zu Radiographie und Tomographie mit thermischen Neutronen und zu deren routinemäßigem Einsatz“ M. Balasko, “Neutron radiography visualization of internal processes in refrigerators”, Physica B: Condensed Matter, Volumes 234-236, 2 June 1997, Pages 1033-1034 M. Balaskó, E. Svab, 1996, „Dynamic neutron radiography instrumentation and applications in Central Europe”: Nucl. Instr. and Meth. A, vol. 377, pp. 140-143 B. Schillinger, T. Bücherl, Neutronen sehen das was Röntgenstrahlen verborgen bleibt ZfP-Zeitung, april 2004, Nr. 89, pages 3441.